Large quantities of oxygen are required for purposes of coal gasification, production of synthetic liquid fuels and in combustion processes involving the use of oxygen. In certain of the foregoing processes, upwards of between 10,000 and 15,000 metric tons per day of oxygen can be consumed.
The cryogenic rectification of air is the preferred method for large scale oxygen production. In cryogenic rectification, air is compressed and purified of higher boiling contaminants such as carbon dioxide, water vapor and hydrocarbons in a pre-purification unit. The compressed and purified air, which in certain plants can be further compressed, is cooled to a temperature suitable for its rectification and then rectified in distillation columns to separate the components of the air. The distillation columns that are employed in cryogenic rectification processes include a higher pressure column and a lower pressure column. In the higher pressure column, the air is rectified to produce a nitrogen-rich vapor column overhead and a crude liquid oxygen column bottoms also known in the art as kettle liquid. A stream of the crude liquid oxygen column bottoms is further refined in the lower pressure column to produce the oxygen product.
Distillation column diameters increase in proportion to the square root of plant capacity or in other words the flow through the columns. Shipping limitations result in a maximum vessel diameter in the range of 6.0 to 6.5 m. As a consequence, the design, construction and installation of an air separation plant having an oxygen production capacity in excess of about 5000 metric tons per day has not been found to be practical. In order to overcome this limitation, typically multiple, parallel air separation plant trains are constructed to operate in parallel within an enclave. Unfortunately simple plant replication forfeits many “economies of scale” in that the construction of additional column shells carries with it considerable expense. Thus, even when multiple air separation units having higher and lower pressure columns are employed within an enclave of such units, it is desirable that each such unit be constructed with the largest capacity possible to limit the number of units employed within a particular installation of air separation plants.
A critical limitation associated with a distillation column involves the hydraulic flood point of any given column section. Column diameters are typically defined by an approach to flood that can be anywhere from 70 to 90 percent. Given equivalent pressure, nitrogen has a lower mass density than oxygen. As the lighter (more volatile) component of air, nitrogen flows to the top of the associated (nitrogen/oxygen) rectification sections. As the column vapor ascends it is progressively enriched in nitrogen. Conversely, the descending liquid becomes richer in oxygen. As a consequence of these thermodynamic aspects, the upper sections of the major low pressure air distillation columns, known as the nitrogen rectification sections, exhibit the highest volumetric loadings. Given a fixed maximum diameter and packing selection, such sections will limit capacity of each plant.
As will be discussed, the present invention provides a method and apparatus by which air separation units can be integrated in a manner that will increase plant capacity and the production of oxygen within plant enclaves having multiple plants.